Solid waste management has long captured the public's attention. As landfills continue to close, new avenues must be opened for recycling municipal solid waste--paper and paperboard products, containers and other packaging.
Molten adhesive compositions, commonly referred to as hot melt adhesives, are normally used in the manufacture and packaging of paper products. Products which have been recycled from such paper products are often characterized by heavy spotting. Such spots are formed when the waste material is heated. As a result, the conversion of municipal solid waste materials containing hot melt adhesives to desirable recycled end-products has been relatively unsuccessful.
Hot melt adhesive particulates have been identified as the source of other problems encountered in mills; such as deposition on wire and press felts and doctor blades; breaks at the wet end or during re-reeling; and product quality deterioration due to sheet holes. Productivity and monetary losses have therefore been high.
The recycling of wastepaper from materials containing hot melt adhesives proceeds generally by one of three methods. In two of the methods, the hot melt adhesive is separated from a water slurry of the recycled fibers using either cleaners or screens. When screens are employed, the pulp slurry is passed through a cylindrical screen plate fitted with small holes or fine vertical slots. The particles of hot melt adhesives are separated out and rejected. When cleaners are used, the pulp slurry is pumped into the cleaner to form a spiraling flow. The slurry is thereby subjected to a centrifugal field. A central core is formed in which the particles of hot melt adhesives concentrate. A portion of the pulp slurry is extracted from this central core carrying the rejected hot melt adhesive particles.
In the third method, a thickened pulp slurry is heated to a temperature, generally ranging from 180.degree. F. to 250.degree. F. It is then subjected to an intensive shearing and/or kneading action. The hot melt particles are broken down to fine sizes, thereby minimizing spotting. Considerable energy is expended, however, in the process.
In the design of modern recycling systems, interest has been centered on those properties of hot melt adhesives which are useful in the separation of the adhesive particulates from the pulp slurry. Much attention has been paid to the size of the hot melt adhesive particulate and its resistance to breakup when the wastepaper is wetted and broken up in a pulper. Pulpers have been specifically designed to minimize breakup of the hot melt particulate during the pumping action. The larger particulates are separated from the pulp slurry with screens. To separate hot melt adhesive particles that pass through the screens, the pulp slurry is pumped into a through-flow cleaner wherein the smaller hot melt adhesive particulates are removed. If screening and cleaning are insufficient to produce the required cleanliness, the pulp is subjected to dispersion.
Molten adhesives must be repulpable in order to maximize wastepaper quality. Hot melt adhesives typically comprise an elastomeric resin and a tackifying resin. Antioxidants, fillers and plasticizers may further be incorporated. The elastomeric resin serves to form the backbone and contributes to the cohesive strength and toughness of the composition. Tackifying resins promote specific adhesion and help the adhesive to wet the substrate. In addition, a wax is frequently added to decrease melt viscosity, increase the crystallinity and control the open and set times of the elastomeric resin.
It is most desired to develop a hot melt adhesive which is compatible with aqueous systems and is recyclable. Compatibility is defined by the ability of the composition to impart adhesive properties to contiguous layers. Compatibility may be measured by various means such as viscosity stability, melt flow characteristics and cloud point. In particular, compatibility references the physical state wherein the elastomeric resin, tackifier and other additives are in a homogenous indistinct blend. In an incompatible adhesive, the elastomer, tackifier and other additives are in discrete areas in the molten state.